This invention pertains to a mining drum assembly for mining a substrate and a method of mining the substrate. More specifically, the invention concerns a drum assembly, and parts of that assembly, for the milling of a roadway substrate to a fine texture. The invention also concerns a method for milling the roadway substrate to a fine texture.
One major component of a road milling machine is the road milling drum. The typical road milling drum of the past comprises a generally cylindrical drum with a plurality of road milling bit-block assemblies directly attached to the surface of the drum. More specifically, the block, which rotatably holds the bit, is welded to the surface of the drum.
The road milling bits are oriented relative to the surface of the drum so that upon the road milling machine powering the drum so as rotate the same the bits impinge upon the roadway substrate and travel through the substrate thereby causing the roadway substrate to disintegrate to a depth equal to the depth of cut for the bit so as to create debris. Typically, the debris is collected and removed from the road milling site. In the case where the roadway substrate is made from an asphaltic material, the debris may be transported to a recycling facility.
The pattern of the road milling bits on the drum is such that each road milling bit impinges upon the substrate at an exclusive discrete point so that the points of impact span the length of the drum. In the past, the typical spacing between the discrete impact points has been about 0.625 inches. While such a spacing of the impact points has been satisfactory for removing the surface layer from the roadway substrate, there have been some undesirable properties of the resultant roadway surface.
Most notably, an impact point spacing of 0.625 inches results in a surface with a coarse texture which leads to a high level of road noise when a vehicle travels over the textured surface. Such a coarse textured surface is irritating to the vehicle driver because of the high noise level and of the fact that the roadway surface is not smooth. The only known way to reduce this road noise from a coarse textured surface is to resurface the roadway with a new layer of roadway material such as, for example, asphaltic material.
Resurfacing the roadway may be acceptable in some circumstances when a resurfaced roadway is necessary. However, when resurfacing is not a necessity, such as in the case where the roadway has been milled to smooth out the surface due to traffic ruts, resurfacing can be an uneconomical approach to solving the problem of a milled roadway with a rough texture.
One approach to solve this problem has been to decrease the impact point spacing so as to make the texture of the milled roadway surface less coarse. While this approach has technical merit, there has been only one manufacturer of road milling drums who has designed a road milling drum with the specific intent to decrease the impact point spacing. In this regard, Keystone Engineering & Manufacturing Company, of Indianapolis, Ind., has designed a road milling bit holder that results in a minimum impact point spacing of about 0.200 inches.
Referring to the design of this holder from Keystone Engineering, it has a rearward shank portion by which the holder is affixed in a pocket of a helical vane on the surface of the drum. The shank terminates at its axially forward end in an enlarged head which has a trio of bores. Each of the bores receives a road milling bit so that the head holds three bits. This holder has a number of drawbacks.
The Keystone Engineering holder is relatively expensive to manufacture. Because of its design and the size of a standard road milling bit, the structure of the current Keystone holder is not conducive to providing impact point spacing below 0.200 inches. Furthermore, while the impact point spacing of 0.200 inches reduces the road noise from surfaces with an impact point spacing of 0.625 inches, there remains a need to decrease even further the impact point spacing so as to produce a milled roadway substrate with a still finer surface texture.
Road milling drums of the past have not been manufactured with modular components. In other words, the road milling drums of the past have been made without regard to using modular pre-manufactured components suitable for use on drums of different designs and bit patterns. By providing a road milling drum made with modular components one would decrease the cost of manufacturing a road milling drum. The use of modular components would also accelerate the time it takes to manufacture a drum, as well as provide for an increase in the design flexibility to make drums of different designs from modular component parts.
Road milling drums must be able to withstand great forces exerted thereon during the road milling operation. To provide any structure that strengthens the road milling drum would be highly desirable.
As can be appreciated, the road milling bits must be changed from time-to-time during the road milling operation since these bits wear out and must be replaced. Although the need to change bits varies with the particular milling conditions, it is not unusual to change bits on a road milling drum at least once per milling shift.
To change a road milling bit, the operator uses a pneumatic hammer to knock the old bit out of the block. Often times there are hundreds of road milling bits on one road milling drum so that the time needed to change an entire drum of bits can be substantial. It would be beneficial to provide a road milling drum that helps the operator gain access to the rear of each road milling bit on the road milling drum.
It is important that the debris generated from the road milling operation be efficiently directed to the location on the milling machine where it is collected and removed from the milling site. It would thus be desirable to provide a road milling drum that enhances the ability of the road milling machine to collect debris for removal from the milling site.